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FUNGUS GNATS, OR SOMETHING WORSE?

By Chris Williams on January 3, 2018.

Lately when we’re watching TV downstairs, we keep seeing these little black gnats flying around or running on top of the side table where we keep potted plants by the window. We can’t figure out where they’re coming from. They’re only in that one room on the lower level of our house. It’s winter so I don’t think they’re coming from outside. What do you think they are? J. P., Concord, MA

Identifying little flies or gnats is tough. We really need to see an actual specimen, or ideally two or more specimens. There are at least 3 types of small flies that can be found indoors, even in the winter: phorid flies, moth (drain) flies, fruit flies, and fungus gnats. Unfortunately, they all look pretty much the same without magnification. They are all associated with decaying organic material of some type and there is some overlap in the preferred sites that they infest. Gnats Indoors in Winter? has a discussion of these tiny flies.

HOUSEPLANTS MAY BE THE SOURCE

However, you mentioned a couple of things that could mean fungus gnats are your problem. Fungus gnat larvae are little whitish worms that feed on fungus and plant roots in wet soil. In homes, they are almost always associated with overwatered houseplants. When the adult flies emerge from the plant’s pot, they don’t travel too far from their breeding site. The fact that the gnats are seen near the houseplants downstairs suggests an association.

These indoor plants may have been infested for some time, or maybe they were just moved inside in the fall after spending a summer outside. Fungus gnats are attracted to light which could also explain why they are near the window. At night they may be attracted by table lamps or even the TV. Fungus gnats also tend to run on surfaces in a jerky manner more than they fly.

OVERWATERING PLANTS MAY BE THE CAUSE

Whether you brought the plants inside or they are new acquisitions, you may be compensating for the drier indoor winter air by watering the plants more than is necessary. As long as the soil remains constantly wet, fungus gnats will continue to develop in the soil, and new adult flies will continue to emerge from the pots. The cure is pretty simple, cut back on the watering and let the soil dry out completely between waterings. This will kill the fungus and the larvae in the pot’s soil (see Fungus Gnats? – Lose the Watering Can).

LET COLONIAL PEST CONFIRM THE DIAGNOSIS

Give Colonial Pest a call and have us inspect your home to rule out other winter gnats. While fungus gnats are fairly easy to deal with, if your flies are phorid flies or moth flies instead, control is more involved. These other little flies can infest multiple sites so you would want us to eliminate them ASAP before they move to other parts of your home.